1. Field of the Invention
The present invention relates to an ink presetting method which adjusts the amount of ink remaining on ink rollers before the next printing operation in an apparatus for offset printing.
2. Description of the Background Art
A typical offset printing apparatus includes a plurality of ink duct (or ink fountain) devices having ink keys, and is capable of supplying variable amounts of ink in a direction crosswise to a predetermined feed direction of a paper sheet to be printed. This controls a distribution of the amount of ink depending on the area of an image on a printing plate in such a manner that an increased amount of ink is supplied to a region having a large image area or consuming much ink whereas a reduced amount of ink is supplied to a region having a small image area or consuming less ink.
In printing operations, it is general to replace printing plates after one printing operation to perform the next printing operation. In this case, there arises a need to readjust the amount of ink supply for each printing operation because of a difference in image to be printed on paper sheets between the preceding and current printing operations.
Unfortunately, the amount of ink remaining on ink rollers after the preceding printing operation is often non-uniform based on the distribution of the image area in the preceding printing operation. It is hence difficult to make a change to a new ink amount distribution in a short time at the beginning of the next printing operation. This consumes a large amount of spoilage (or many waste paper sheets) before the stabilization of the quality of the printed sheets at the beginning of the next printing operation, and thus requires much time.
A solution to the above-mentioned problem includes a known technique disclosed, for example, in Japanese Patent Application Laid-Open No. 10-16193 (1998). In this background art technique, printing is performed with the supply of ink suspended to reduce the ink remaining on the ink rollers to a minimum required amount. That is, this technique transfers ink to the paper sheets to eliminate the irregularities of the distribution of the amount of ink on the ink rollers resulting from the preceding printing operation to provide a uniform distribution. Then, new ink is supplied before the next printing operation, and ink presetting is completed.
This background art technique is effective to provide uniformity of the amount of ink distributed on the ink rollers, but is disadvantageous in requiring waste paper sheets (or spoilage) to be used until the removal of ink. In particular, if an image having a relatively low density in one printing operation follows an image having a relatively high density in its preceding printing operation, a large amount of ink has been supplied onto the ink rollers in the preceding printing operation. This necessitates a large amount of ink to be removed, to require relatively large amounts of time and spoilage for completion of ink presetting.
The present invention is intended for a method of presetting ink in a printing apparatus, the apparatus comprising an ink supply mechanism capable of variably supplying ink, and an ink transfer mechanism capable of receiving the ink from the ink supply mechanism and transferring the ink onto a plate cylinder provided to print an ink image on a printing sheet being fed in a predetermined feeding direction. According to the present invention, the method comprises the steps of: a) setting a first target density for a plurality of ink key regions defined on each printing sheet along the feeding direction; b) performing a first printing operation under a first feedback control of the ink supply mechanism using the first target density; c) performing a second printing operation under a second feedback control of the ink supply mechanism using a second target density lower than the first target density after a required number of sheets are printed in the step b); and d) when a printed sheet has a density lowered to the second target density, judging that the amount of ink remaining on the ink transfer mechanism is equal to an amount required to restart the printing apparatus for a next printing operation, thereby to stop the second printing operation.
This provides the uniform printed density to provide a substantially uniform amount of ink remaining on the ink rollers, thereby facilitating the start of the next printing operation. In particular, when reducing the amount of ink on the ink rollers by transferring ink to a printing plate surface and a blanket plate, there is produced an additional effect of eliminating the need for spoilage for adjustment of the amount of ink.
Preferably, the respective densities on the plurality of ink key regions are measured with a density detector provided in the printing apparatus.
Preferably, the respective ink keys of are adjusted so that the second values of measured density are within a xc2x10.2 range around the second target density in the step c).
The present invention is also intended for a method of presetting ink in a printing apparatus, the apparatus comprising an ink supply mechanism capable of variably supplying ink, and an ink transfer mechanism capable of receiving the ink from the ink supply mechanism and transferring the ink onto a plate cylinder provided to print an ink image on a printing sheet being fed in a predetermined feeding direction. According to the present invention, the method comprises the steps of: a) setting a target density for a plurality of ink key regions defined on each printing sheet along the feeding direction; b) performing a printing operation under a feedback control of the ink supply mechanism using the target density; c) changing the printing apparatus to the intermediate state where transfer of the ink from the ink supply mechanism to the ink transfer mechanism is stopped and the printing operation is also stopped; d) transferring residual ink on the ink transfer mechanism to the plate cylinder in the intermediate state; and e) stopping the printing apparatus.
It is therefore an object of the present invention to provide a novel method of presetting ink which previously eliminates the irregularities of a distribution of the amount of ink on ink rollers.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.